I have always liked this line from his poem "The Ladder of St. Augustine"
The heights by great men reached and keptWere not attained by sudden flight,But they, while their companions slept,Were toiling upward in the night.
As a teacher and a coach, I simply can not stress enough the importance of hard work. Sure some players and students are naturally gifted far beyond the rest of their teammates and/or classmates. The problem I have seen with that though is that they tend to make some of the laziest players and students that I work with. They seem to believe that their natural abilities will always see them through the most difficult adversities. And they are wrong. They are wrong, because they have never learned and practiced how to overcome challenges through dogged determination. There is no substitute for hard work in my opinion, and Longfellow's quote echoes that opinion of mine. He says that great men are great because they work harder and longer than their peers. The quote is very similar to this Thomas Edison quote:
Genius is one percent inspiration and ninety-nine percent perspiration.
Or this quote from Steve Prefontaine:
To give anything less than your best, is to sacrifice the gift.